The present invention relates to laser processing of a workpiece, and, more particularly, to determining breakthrough during drilling using a laser.
Of the three primary laser processing activities, namely cutting, welding, and drilling, drilling presents the most difficult control problem. Consider, for example, laser hole drilling of an aircraft engine combustor and afterburner parts. These parts are made from high temperature steel alloys and require tens of thousands of 0.020 inch (0.0508 cm) holes drilled at 20 degrees to the surface, where wall thickness may vary from 0.020 inch (0.0508 cm) to 0.080 inch (0.2032 cm). There are at present only two viable techniques for sensing hole properties (diameter, shape, recast layer thickness, etc.) for process control, namely, air flow testing and pin checking.
In air flow testing, the workpiece is removed from the drilling apparatus and a known pressure differential is applied across the workpiece. The resulting air flow is measured to give a measure of the flow resistance. In turn, this gives a measure of the drilled area, i.e., the diameter of the drilled holes and their shape as the number of holes drilled is known. This method is quite reliable for average hole diameter and shape, but only for fairly large ensembles of holes, and is not real-time in the sense that laser processing can take place while the flow test is being made. It is not a reliable indicator of other geometric properties, e.g. recast layer thickness, hole taper, etc.
In pin checking, the drilling is stopped, and then pins of successively increasing diameter are successively inserted into the holes. Pin checking is only approximate as an indicator of hole diameter because laser-drilled holes are rarely very straight, thus blocking insertion of the pins. It is also not a reliable indicator of other geometric properties nor is it a real time process.
While optical techniques might be used, they are unreliable, due to the angle involved and the length of the holes. Further, the large number of holes makes optical monitoring difficult.
One expected indirect indicator of the properties, such as diameter, of a laser-drilled hole is the "breakthrough time", or the fractional time to breakthrough as compared with the total laser pulse length. The longer the breakthrough time, the smaller the hole diameter because after breakthrough the laser light pulse enlarges the drilled hole.
It is therefore an object of the present invention to monitor a laser processing system, and particularly to determine breakthrough time during laser drilling.